A roll caster typically suffers from two types of sticking during the metal forming process, the first is "severe sticking" and the second is "microsticking."
Severe sticking occurs when the surface of the metal strip does not release from the rolls. In this case, the cast metal shears internally to maintain the bulk extrusion of the metal and the surface of the strip is torn away from the roll surface at the exit of the rolls. Severe sticking is very detrimental to both product quality and the operation of the machine. The metal flow mechanisms in the roll bite are such that, in normal operation, the strip can only "stick" to one roll at a time.
To help prevent the onset of the sticking phenomenon, the caster rolls are normally continuously "painted" or sprayed with a graphite lubricant. This lubricant is typically applied with a single spray nozzle or a "bar nozzle" in which numerous "nozzle" holes are present in a single tube. In small quantities, this lubricant assists in the release of the cast metal from the rolls. However, it is important that a correct amount of graphite be applied to the caster rolls.
If too much graphite is applied to one of the rolls, surface defects may appear on the metal cast strip. The surface defects are caused by a reaction between the metal strip and the surface of the rolls which is coated with an aluminum/aluminum oxide coating. The aluminum/aluminum oxide coating on the rolls absorbs the water from the water-based parting agent solution. The absorption of the water is thermally reversible. Thus, when the roll coating comes into contact with the molten metal at the roll nip the water vapor and hydrogen that was previously absorbed is released in the roll nip. This release of water causes tip material degradation, a loss of caster productivity, and minor surface defects on the metal strip.
If too little graphite is applied to one of the rolls, the caster does not enter the sticking condition immediately, but operates for one or more revolutions, consuming the graphite that has accumulated on the surface of the roll. Eventually, however, there is insufficient graphite for the caster to operate in a stable manner, and the strip begins to stick to the graphite-deficient roll. This initial sticking consists of a series of sticks and releases which is referred to as "microsticking." Microsticking can be seen on the strip surface as transverse bands varying in length in the casting direction from 0.25" to 2". This translates into a stick and release frequency of 4 Hz to 0.5 Hz at a casting speed of 60 IPM or 8 Hz to 1 Hz at 120 IPM. As the casting speed further increases, so does the stick and release frequency.
The stick and release phenomenon of microsticking for most cast metal is not tremendously harmful to product quality. Unfortunately, when the areas of the roll surface that have experienced microsticking rotate back into the roll bite, they have no remaining graphite coating and cause continuous severe sticking to occur.
Applying the critical amount of graphite is dependent on the skill of the operator, who must carefully apply enough graphite to prevent both severe sticking and microsticking. In addition, the operator must carefully watch the entire casting cycle because the graphite tends to plug up the graphite spray nozzles. Once the spray nozzles begin to plug up, an insufficient amount of graphite is applied to the rolls and microsticking once again begins to occur.
It is extremely inefficient to rely on an operator to watch the entire casting process and look for signs of microsticking which occurs when the spray nozzles begin to plug up. Further, in order to supply sufficient graphite to the rolls when graphite spray nozzles begin to clog, the operator will increase the amount of spray that is being supplied to the spray nozzles. However, once the nozzles become severely clogged, any increase in the amount of graphite supplied to the spray nozzles will only result in additional clogging. At that time, the casting machine must be halted and the spray nozzles cleaned. Shutting down the machine to clean the spray nozzles, however, seriously reduces the overall productivity of the roll caster.
In the past, when microsticking or severe sticking occurred and adjustments to the amount of graphite which was supplied to rolls did not help, the operator would slightly slow the casting machine to increase the amount of extrusion. However, slowing the casting machine is counterproductive to the ultimate goal of improving the yield of the casting process.
Additionally, with the increasing speeds that are achievable with the latest roll caster designs, greater amounts of parting agents must be applied to the rolls of the roll caster to prevent sticking conditions from occurring. As the amount of parting agent solution applied to the rolls increases, the amount of "over spray" that remains in the surrounding environment increases. The presence of excess amounts of graphite solution in the surrounding environment may result in the improper operation of certain types of magnetic based equipment. In addition, the presence of excess amounts of graphite solution in the surrounding environment may also be detrimental to the health of the employees.
Therefore, there exists a need in the prior art for an improved parting agent spray system that provides a roll casting machine with increased levels of a parting agent while reducing the problems associated with excess levels of parting agents contaminating the surrounding environment.